1. Field of the Invention
The present invention relates to a heat-sensitive recording material superior in image stability and good in heat responsiveness.
2. Related Art
Heat-sensitive recording materials generally comprise a support and, provided thereon, a heat-sensitive recording layer mainly composed of an electron donating colorless dye precursor and an electron accepting developer. Upon application of heat to the heat sensitive recording layer by a thermal head, hot pen, laser beam, and the like, the colorless dye precursor instantaneously reacts with the developer to form record images. These are disclosed in Japanese Patent Kokoku Nos. 43-4160 and 45-14039.
These heat-sensitive recording materials have such merits that record can be obtained by relatively simple devices and thus maintenance is easy and no noise is generated. Therefore, they are widely used in recorders for instruments, facsimile, printer, computer terminal equipment, labeling, ticket vending machines, and the like.
These heat-sensitive recording materials comprising electron donating colorless dye precursor and electron accepting developer have the excellent properties that they are good in appearance and touch and can provide high color density and various hues while they have the defects of inferior record stability, for example, theremally colored portion (recorded image portion) disappears due to plasticizers or additives contained in plastics upon contact with plastics such as polyvinyl chloride or readily disappears upon contact with foods or cosmetics or easily discolored upon exposure to sun light for a short time. Owing to these defects, their use is respected and solution of these defects have been strongly demanded.
As heat-sensitive recording materials which can form record images of good storage stability by reaction of two components upon application of heat, those which comprise an imino compound and an isocyanate compound as the two components are disclosed in Japanese Patent Kokai Nos. 58-38733, 58-54085, 58-104959, 58-149388, 59-115887 and 59-115888 and U.S. Pat. No. 4,521,793.
The heat-sensitive recording materials comprising isocyanate compound and imino compound being excellent in record stability are excellent in stability of image, but have the problem that they are insufficient in heat responsiveness and thus record images of sufficient density cannot be obtained by high-speed printing device.
For improvement of heat responsiveness, use of 2-benzyloxynaphalene or di(4-chlorobenzyl) oxalate has been proposed, but at present, higher heat responsiveness has been desired.